Washington (CNN) -- Sen. Arlen Specter's decision to leave the Republican Party in April handed President Obama a key vote in the Senate, and Specter was rewarded by quickly being endorsed by the president and Democratic leaders in his bid for re-election next year.

But not every Democrat got in line behind Obama.

In Kentucky, GOP leaders spent the first half of the year hoping that Sen. Jim Bunning would retire, thereby allowing a more popular Republican to run and giving the GOP a better chance to hold onto the seat.

For months, Bunning refused to say he was retiring, but by midsummer the Hall of Fame baseball pitcher relented and provided Republican leaders an opening to rally around the candidacy of Kentucky Secretary of State Trey Grayson.

But not every Republican got in line behind the GOP establishment.

Primaries are complicating matters for Democratic and Republican Senate leaders, who would prefer to focus all of their attention on the 2010 general election, not internal battles for party nominations. Republicans have more primaries to sort out than Democrats next year, but Democrats have their own set of issues to address in the midterm elections.

A lot is riding on the 2010 Senate contests: Obama's future, the Democratic Party's future, the Republican Party's future, and the political careers of lawmakers who have made the Senate their home and those who want to call the "World's Most Exclusive Club" home.

Heading into 2010, Democrats must defend 19 seats, which includes an opening that occurred when Massachusetts Sen. Edward Kennedy died. A special election for Kennedy's seat will take place in January. Republicans must protect 18 seats. And Democratic and Republican strategists are quick to point out opportunities to pick up seats that the other party now controls.

In the Senate, the magic number is 60. It takes 60 senators to break a filibuster. Thanks to Specter's defection and the fact that two independents align themselves with the Democratic Party, Democrats control 60 votes. In theory, Obama should be able to marshal his legislative agenda through the chamber. But as we are witnessing in the debate over health care reform, 60 is just a theoretical number, and for the president to overcome obstacles in the Senate -- yes, even hurdles erected by members of his own party -- he needs more than 60 votes to win on the more contentious issues.

We were flying into a headwind in December [2008], now the headwind is at our back.--Sen. John Cornyn, National Republican Senatorial Committee chairman

The task of getting Democrats electedfalls to Sen. Robert Menendez, D-New Jersey, who serves as chairman of the Democratic Senatorial Campaign Committee in this two-year election cycle. "We are going to do everything to preserve our present majority and try to build on it," he said in an interview.

While the chairmanship of a political party committee can be a thankless task -- frequent travel and constant fundraising -- it appeared at the beginning of the year that Menendez would have an easy job. Democrats had just picked up at least seven seats, Obama's approval rating was soaring, and in the reliably Republican state of Georgia, Sen. Saxby Chambliss needed a runoff to win re-election.

Conversely, Republicans were down in the dumps. "Despondent is probably a better word," to describe the GOP's mood, said Sen. John Cornyn, R-Texas, chairman of the National Republican Senatorial Committee, Menendez' political counterpart.

"We had just been beaten very badly in a national election that depleted our numbers in the Senate and House and elected Barack Obama to the White House," Cornyn said.

To make matters bleaker for the GOP, Specter switched parties, and the unresolved Senate race in Minnesota was finally called in favor of Democrat Al Franken. Democrats had the 60 votes they had coveted.

But as the summer wore on, the future did not look as dark for Republicans. Obama's soaring favorability rating dropped -- it has now settled back to an earthly 55 percent. The Democratic Party's signature domestic issue, health care reform, was in a critical state, and Republicans finally settled on a message of fiscal responsibility.

Then the GOP got some additional good news, when cracks in the veneers of two Senate veterans were exposed. Early polling showed that Majority Leader Harry Reid, D-Nevada, and Sen. Chris Dodd, D-Connecticut, were in danger of losing re-election battles in 2010. And additional surveys indicated that Republicans had a legitimate chance of picking up the seats once held by Obama and Vice President Joe Biden.

"We were flying into a headwind in December, now the headwind is at our back," Cornyn said.

[Republicans] have adopted a strategy that has really put them in a bad position.--Sen. Robert Menendez, Democratic Senatorial Campaign Committee chairman

A bit of an overstatement, perhaps, by Cornyn, but it is hard to dispute that the political landscape looks much better now for the GOP than it did in January. Still, Republicans face their own set of issues.

While the GOP establishment was happy to see Bunning step aside in Kentucky, Cornyn would have liked to see Republican Sens. Kit Bond of Missouri, Judd Gregg of New Hampshire, and George Voinovich of Ohio stay put. But each is leaving when the curtain drops on the 111th Congress, providing Democrats with opportunities to take those seats.

When it comes to early endorsements, Florida has become ground zero for Cornyn's primary problem. Republicans quietly breathed a sigh of relief when Sen. Mel Martinez -- who showed little enthusiasm to run for another term -- announced last year that he would retire at the close of his term in 2010. He has since resigned his seat, and Gov. Charlie Crist appointed his longtime political aide George LeMieux to temporarily fill it until the next election.

Martinez' decision to leave opened the door for the popular Crist, a fellow Republican, to run for his own six-year Senate term. But many conservative activists are no fans of Crist, who they claim is not conservative enough for their liking. These conservatives are pinning their hopes on the candidacy of former House Speaker Marco Rubio, who has refused to bow to pressure to abandon his primary challenge.

Cornyn came under fire from these conservatives for endorsing Crist in the primary, a backlash that has given the NRSC pause in endorsing candidates in other contests.

Cornyn defended his decision to endorse Crist, who he emphasized is well-liked throughout the state and is able to raise the necessary amount of money to run a successful campaign.

"It was something I needed to do, because he felt so strongly about it," Cornyn said. But Cornyn added that in other primaries, such as New Hampshire, the NRSC has no plans to back a candidate.

Former state Attorney General Kelly Ayotte is the favorite of national Republicans in New Hampshire, yet she faces a primary; in Kentucky, Rand Paul -- an ophthalmologist and son of Texas Rep. and presidential candidate Ron Paul -- is taking on Grayson; in Illinois, Rep. Mark Kirk faces a challenge from the more conservative wing of his party; and in California, Colorado, Connecticut and Nevada, Republicans need to settle on a candidate before setting their sights on the general election.

Cornyn's Democratic counterpart, Menendez, said things look good for Democrats from where he sits.

"I would rather be in my position, where a lot of my incumbents are running for re-election and my appointees are doing exceptionally well," he said. "An open seat is always a greater opportunity for a pickup."

Among those Menendez is trying to get re-elected are Reid, Dodd, and Republican-turned-Democrat Specter.

To say that Specter has been embraced by the Democratic establishment is an understatement, as Obama and Biden have both campaigned on his behalf. But Rep. Joe Sestak, a former admiral, is not backing down from running against Specter. He hopes to tap into a frustrated Democratic base, which has been voting against Specter since he was first elected to the Senate as a Republican in 1980.

And Menendez must spend money and resources on trying to keep Obama's Illinois seat and Biden's Delaware seat in Democratic hands. A handful of Democrats are running for the open seat in Illinois. In Delaware, Democratic leaders are waiting to see if the vice president's son, Beau, will announce his candidacy for his father's old Senate seat. Democrats see hope in Kentucky and Ohio, but the party must first address primaries in each of these states.

As for which party has the momentum heading into 2010, it comes as no surprise that Menendez and Cornyn offer competing perspectives.

Menendez acknowledged that the GOP was able to capitalize politically on the issue of health care when it flared up this summer. "When they were aggressive in the summer, we were not as aggressive and they really gained some momentum," he said.

Yet Menendez contended it was a short-term gain that will have long-term negative consequences for the GOP. "They have adopted a strategy that has really put them in a bad position," Menendez said. "They are going to do everything to be the 'Party of No' and obstruct. Having the president fail really that means the country fails."

Cornyn sees it differently and said that Democrats have overplayed their hand.

"They misinterpreted what the mandate was," Cornyn said of the 2008 election. "They somehow assumed that their election was a mandate for change in ideology, hard to the left. People around the country ... are concerned about the borrowing and spending and the government takeovers."

While these two men will be focused on the 36 Senate contests in 2010, another interesting storyline comes only two years later. In 2012, Democratic leaders must defend the seats of 21 Democrats and two independents, while Republican leaders only have to protect nine senators.